1. Field of the Invention
The present invention relates in general to field emission display devices used in flat panel displays .and a method for producing such display devices and, more particularly, to improvements in a structure of the field emission display device and in the production method of the display device for forming, without using a micro gate aperture forming technique, a field emission cathode, which cathode is necessary in production of a large-sized display device, into a structure suitable for lengthening the expected life span of the field emission cathode, thereby simplifying the production process and reducing the cost of the display device.
2. Description of the Prior Art
Known picture displays include cathode ray tubes (CRT) and flat panel displays, such as liquid crystal displays (LCD), plasma displays (PDP) and vacuum fluorescent displays (VFD), which flat panel displays are recently in the limelight of the display field.
The CRTs have somewhat improved efficiency in view of picture quality and luminance, nevertheless has a problem that their volume and weight are remarkably increased when increasing their sizes. This directly runs counter to the recent trend of compactness, lightness, thinness and diminution of the displays. On the contrary, the flat panel displays such as LCDs, PDPs and VFDs are more advantageous in view of the volume and weight in comparison with the typical CRTs. However, such flat panel displays have a problem that their picture quality and luminance are inferior to those of the CRTs. In recent years, field emission display devices have been actively studied and developed in order for providing the good picture quality of the typical CRTs and the structural advantage of the typical flat panel displays for the display devices at the same time. The field emission display devices are produced by precise machining of field emission cathodes in accordance with advance of semiconductor technique and by use of the field emission cathodes in the display devices.
Typically, the field emission display devices are produced by Spindt process which is a representative process for production of the field emission display devices. The most important techniques of the above Spindt process are techniques for forming a cathode tip having submicron apex and for forming gate apertures formed in a gate electrode.
FIG. 1 is a sectional view of a typical field emission display device having field emission cathodes. As shown in this drawing, both an insulating layer 3 and a gate electrode 4 are orderly formed on a cathode layer 2 of a substrate 1 and a plurality of circular pattern apertures are formed in both the insulating layer 3 and the gate electrode 4. Thereafter, a plurality of conical cathodes 10 are formed in the apertures of both the insulating layer 3 and the gate electrode 4 respectively.
In FIG. 1, the reference numeral 5 denotes a vacuum region, 6 denotes a fluorescent layer, 7 denotes an anode layer, 8 denotes a face plate glass and 9 denotes a partition.
In process for producing the above field emission display device, the circular pattern gate apertures may be formed through a photolithography. However, as H-rays having a wavelength of about 0.4 .mu.m are used as ultraviolet rays of a pattern exposure system for the photolithography, the minimum pattern size formed through the photolithography is limited to about 1.5 .mu.m and this causes many problems in forming of the gate apertures through the Spindt process. In addition, production of large-sized field emission display devices should be accompanied with development of a new lithography system and this restricts development of the large-sized field emission display devices. In the case of micro-tip cathode, the tip apex of the cathode has a radius of about 50 nm, so that the tip apex may be broken when there is an ion bombardment of gases remaining in the vacuum region due to electrons emitted from the cathode and this makes the microtip cathode short-lived.